This invention relates to a die cast method for manufacturing die cast products of austenite type stainless steel having low melting point.
Generally, stainless steels are classified in a range of from SCS 12 to SCS 23 as prescribed by the Japanese Industrial Standard (JIS) according to this compositions and used for die casting, but these stainless have a high melting point of over 1450.degree. C., so that it is difficult to use these stainless steels for the die casting as they are. For this reason, stainless steels of low melting point have been developed wherein elements such as copper (Cu), manganese (Mn), silicon (Si), boron (B), niobium (Nb), phosphors (P), molybdenum (Mo), and the like are added to the stainless steels to lower this melting point. However, such stainless steel tends to form a boron compound, a phosphor compound and the like compounds which cause cracking of the cast products at elevated temperatures, when they solidify in a metal mold.
The reason for causing the cracking will be described hereunder in conjunction with FIGS. 1a and 1b.
Referring to FIGS. 1a and 1b, the molten metal of a stainless steel poured into in a metal mold A firstly solidifies near the contacting surface to the mold A. The solidified portions B grow as the temperature of the molten stainless steel lowers, and liquid portions C are surrounded by these solidified portions B as shown in FIG. 1a. When the solidification further proceeds, abnormal tension stress is generated on the surface of the cast product in the mold including a core, and cracks are formed at the boundary D between the liquid and the solidified portions. This is the most significant problem in the die casting.